Despite the increasing need for tertiary prevention programme interventions, knowledge is lacking on which methods used in programmes work and which do not. Through semi-structured interviews with twelve practitioners and combination of grounded theory and reflexive thematic analysis, this article explores: how do tertiary prevention intervention providers measure the progress or regression of participants; how do tertiary prevention intervention providers measure the impact of programmes; and what is the reasoning for not practising impact assessment. The results show that 50% of the interviewed practitioners measure participant progress with standardised tools or set criteria. To measure participant progress, practitioners employ screening, needs assessment, indicators of change, and standardised tools. Programme development is tracked through monitoring or evaluation conducted by an outside evaluator or funder. Intervention providers prefer not to conduct impact assessment for various reasons, including their academic background that pushes them away from available instruments, feeling that currently available tools are not reliable or a tendency to work based on professional intuition. They hesitate to guarantee results due to fear of losing funding or credibility if a participant relapses and re-engages. Last, this article brings out practitioners’ perspectives that highlight not only current practices but also several gaps in the field that need further research.